Clostridium difficile (CD) is a Gram positive, spore-forming, anaerobic bacillus recognized to cause diarrhea, pseudomembranous colitis, toxic megacolon, other gastrointestinal conditions, and even death. Hedge et al. (2008) New advances in the treatment of Clostridium difficile infection (CDI), Therapeutics and Clinical Risk Management 4(5):949-64; Tsutsumi et al. (2014) Progress in the Discovery of Treatments for C. difficile Infection: A Clinical & Medicinal Chemistry Review, Current Topics Med. Chem. 14:152-75. In its spore form, the bacterium is able to survive harsh environments and common sterilization techniques. Spores of CD are resistant to high temperatures, ultraviolet light, harsh chemicals, and many antibiotics, and remain viable for months or longer (Leffler & Lamont (2009) Treatment of Clostridium difficile-Associated Disease, Gastroenterol. 136:1899-1912).
Acquisition of CD occurs by ingestion of the acid-resistant spores, which pass through the stomach and germinate and inhabit the colon. In order for CD to over-populate the colon, there is typically a disruption of the normal bacterial flora which otherwise provides colonization resistance to CD and other opportunistic pathogens. As such, the protection of normal gastrointestinal flora is an important defense to CD infection (CDI). Unfortunately, exposure to common antibiotic regimens (e.g., cephalosporins, penicillins and fluoroquinolones) in the course of medical treatment of infectious and other diseases (e.g., such as bowel surgery and/or cancer chemotherapy) disrupts the natural gut flora, allowing CD resistant to such antibiotics to colonize and overpopulate in the gut of some patients, particularly elderly patients.
Once colonized, CD reproduces and releases enterotoxin (toxin A or TcdA) and cytotoxin (toxin B or TcdB) in the colon. Tsutsumi, 2014; Heinlen & Ballard (2010) Clostridium difficile Infection, Am. J. Med. Sci. 340(3):247-52; Hedge et al., 2008. Both toxins A and B act as glucosyltransferases, inactivating small cellular GTPases (Rho, Rac & Cdc42), and triggering the attraction and adhesion of neutrophils resulting in inflammation of the mucosal lining, cellular necrosis, and increased peristalsis and capillary permeability, thereby degrading the colonic epithelial cells and leading to the clinical symptoms associated with CDI. Although both toxins are cytotoxic and stimulate apoptosis, toxin B is more potent while toxin A stimulates epithelial cell permeability and inflammatory response (effecting cytokine, chemokine and reactive oxygen intermediate production; neutrophil infiltration; mast cell accumulation; and substance P production, stimulating submucosal sensory neurons).
The mechanisms of action of CD toxins TcdA and TcdB are illustrated in FIG. 1, plates A and B. See Voth & Ballard (2005) Clostridium difficile Toxins: Mechanisms of Action and Role in Disease, Clin. Microbiol. Rev. 18(2):247-63. Clinical symptoms vary from asymptomatic colonization or mild diarrhea to life threatening illness, including severe inflammation, lesions and/or tissue necrosis of the gut mucosa. Typically, CDI-associated disease begins as watery diarrhea and progresses to pseudomembranous colitis. Distension of the colon may result in toxic megacolon. In addition, even cases of relatively mild CDI may rapidly progress to fulminant CDI, with such patients suffering systemic toxicity (e.g., leukocytosis, hypotension, renal failure, respiratory distress, or even death).
CD is a leading cause of hospital-acquired diarrhea, infections and other disease in Europe and North America. Heinlen & Ballard, 2010. Because of their increasing resistance to many common antibiotics, CD spores can remain in the gastrointestinal tract and potentially contribute to recurrent disease following conventional treatment regimens. As such, CD infections (CDIs) are increasing worldwide and have become more severe and refractory to treatment in the past decade. CDIs are one of the most common nosocomial infections in the United States, and presently, hospital-acquired CDIs exceed that of methicillin-resistant Staphylococcus aureus (MRSA) infections in some regions.
Increased incidence of hospital-associated CDI may be due to the utilization of a broad spectrum of antimicrobial agents to treat various other conditions. As such, the emergence of hypervirulent epidemic strains of CD (e.g., the fluoroquinolone resistant hypervirulent CD strain designated as NAP1/BI/027 in North America and Europe) produce increased amounts of toxins A and B, and thus exhibit increased rates of recurrence, morbidity and/or mortality (as compared to previously identified strains of CD), particularly in vulnerable elderly patients. For example, NAP1/BI/027 produces 16 times more toxin A and 23 times more toxin B compared to many conventional CD strains, and also produces an additional toxin known as binary toxin (see Hedge et al., 2008), which causes a 3-fold higher mortality rate as compared to patients infected with less virulent strains. Leffler & Lamont, 2009.
Effective treatment of CDI has proven to be quite challenging due to the need for selective eradication of CD without affecting the normal gut flora. After each episode or infection, the risk of recurrence of CDI generally increases. About 15%-30% of CDI patients get another infection after their first CDI, and about 40%-60% get a further CDI after their second episode of the infection. Tsutsumi et al., 2014. As such, for patients who experience two or more recurrences of symptomatic CDIs, a change in strategy is warranted. Leffler & Lamont, 2009.
Current therapies for CDI typically provide for the administration of antimicrobial agents including metronidazole (MET), vancomycin (VAN) or fidaxomicin (FDX). MET is the conventionally accepted agent for treating the first episode of CDI due to its relatively low cost, as well as concerns of emergence of VAN resistant enterococci (VRE) by extensive VAN treatment. See Leffler & Lamont, 2009. However, only VAN and FDX have been approved by the U.S. Food and Drug Administration (FDA) for the treatment of recurrent CDI. Moreover, use of VAN and FDX is relatively limited, particularly for treating the first episode of CDI, due to their relatively high cost (e.g., current cost is about $2000 per treatment).
Failure to respond to MET has become more common recently, and therefore VAN has become more commonly used as a first episode treatment, particularly in patients with more severe CDI. However, reports describing MET failures and questions of the equivalence of MET and VAN for CDI have been raised. Hedge et al., 2008.
Recurrence or failure rate associated with conventional MET, VAN and FDX treatments, as well as other conventional therapies, is relatively high. For example, between about 15% and about 30% of patients have a relapse of symptoms after successful initial treatment with such conventional treatments, usually within the first few weeks after treatment is discontinued Leffler & Lamont, 2009. Such relapse is sometimes due to the persistence of the same CD strain being treated, or alternatively or additionally due to reinfection with a new CD strain. With the emergence of hypervirulent epidemic strains of CD, recurrence rates of CDI have jumped to nearly 50% in patients treated with MET and VAN. The spectrum of activity for FDX is narrower compared to that of MET or VAN. However, efficacy of FDX is similar to that of VAN. When used to treat previous strains of CD, FDX exhibits a somewhat lower recurrence or failure rate as compared to MET and VAN. However, FDX exhibits no improvement in recurrence rates in patients infected with hypervirulent strains of CD (Louie et al. (2011) Fidaxomicin versus vancomycin for Clostridium difficile infection, N. Engl. J. Med. 364(5):422-31), and thus also exhibits recurrence rates of nearly 50%. As such, conventional FDX therapies have not proven to be effective for patients that have suffered multiple CDI relapses.
Thus, due to the increasing emergence of resistant and hyper-virulent CD strains and the high rate of recurrence of CDI after treatment with such conventional agents, there is an urgent need to develop new and better therapies. Alternative non-antibiotic based treatments for treating CDI have emerged, including fecal transplants and probiotics, which seek to restore or repopulate the normal gut flora. Other treatment methods, such as immunotherapy-based treatments and vaccines, attempt to neutralize the CD toxins or enhance the patient's immune response. However, such non-antibiotic based treatments have exhibited mixed results with less efficacy as compared to conventional microbial agents. As such, conventional treatments using MET, VAN and FDX remain prevalent.
Thus, new therapeutics and treatment methods are needed to improve efficacy and reduce failure and/or recurrence rates of CDIs. The ideal therapeutic agent would specifically eliminate or substantially reduce a CD population with minimal disturbance to normal gut flora. However, due to the exorbitant costs involved in the discovery and development of new agents, many pharmaceutical companies are reluctant to invest in new drug discoveries. Moreover, the return on investment for drug companies on short duration therapeutics like antimicrobial agents is not alluring compared to long duration or life-long therapies. Consequently, the discovery of new uses for previously known drugs (repurposing) is a very cost effective option. Indeed, the National Institutes of Health has an ongoing initiative in collaboration with pharmaceutical companies for discovering new therapeutic uses for existing molecules.